Processing color and panchromatic pixels

ABSTRACT

A method for forming a final digital color image includes capturing an image using an image sensor having panchromatic pixels and color pixels corresponding to at least two color photoresponses; providing from the captured image a digital panchromatic image and an intermediate digital color image; and using the digital panchromatic image and the intermediate digital color image to provide the final digital color image.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is related to vending U.S. Ser. No. 11/191,729filed Jul. 28, 2005 (now U.S Patent Application Publication No.2007/0024931)of John T. Compton and John F. Hamilton, Jr., entitled“IMAGE SENSOR WITH IMPROVED LIGHT SENSITIVITY”.

FIELD OF THE INVENTION

This invention relates to processing digital images having color andpanchromatic pixels.

BACKGROUND OF THE INVENTION

An electronic imaging system depends on an electronic image sensor tocreate an electronic representation of a visual image. Examples of suchelectronic image sensors include charge coupled device (CCD) imagesensors and active pixel sensor (APS) devices (APS devices are oftenreferred to as CMOS sensors because of the ability to fabricate them ina Complementary Metal Oxide Semiconductor process). Typically, theseimage sensors include a number of light sensitive pixels, often arrangedin a regular pattern of rows and columns. For capturing color images, apattern of filters is typically fabricated on the pattern of pixels,with different filter materials being used to make individual pixelssensitive to only a portion of the visible light spectrum. The colorfilters necessarily reduce the amount of light reaching each pixel, andthereby reduce the light sensitivity of each pixel. A need persists forimproving the light sensitivity, or photographic speed, of electroniccolor image sensors to permit images to be captured at lower lightlevels or to allow images at higher light levels to be captured withshorter exposure times.

Image sensors are either linear or two-dimensional. Generally, thesesensors have two different types of applications. The two-dimensionalsensors are typically suitable for image capture devices such as digitalcameras, cell phones and other applications. Linear sensors are oftenused for scanning documents. In either case, when color filters areemployed the image sensors have reduced sensitivity.

A linear image sensor, the KLI-4104 manufactured by Eastman KodakCompany, includes four linear, single pixel wide arrays of pixels, withcolor filters applied to three of the arrays to make each arraysensitive to either red, green, or blue in its entirety, and with nocolor filter array applied to the fourth array; furthermore, the threecolor arrays have larger pixels to compensate for the reduction in lightsensitivity due to the color filters, and the fourth array has smallerpixels to capture a high-resolution monochrome image. When an image iscaptured using this image sensor, the image is represented as ahigh-resolution, high photographic sensitivity monochrome image alongwith three lower resolution images with roughly the same photographicsensitivity and with each of the three images corresponding to eitherred, green, or blue light from the image; hence, each point in theelectronic image includes a monochrome value, a red value, a greenvalue, and a blue value. However, since this is a linear image sensor,it requires relative mechanical motion between the image sensor and theimage in order to scan the image across the four linear arrays ofpixels.

There is also known in the art an electronic imaging system described inU.S. Pat. No. 4,823,186 by Akira Muramatsu that comprises two sensors,wherein each of the sensors comprises a two-dimensional array of pixelsbut one sensor has no color filters and the other sensor includes apattern of color filters included with the pixels, and with an opticalbeam splitter to provide each image sensor with the image. Since thecolor sensor has a pattern of color filters applied, each pixel in thecolor sensor provides only a single color. When an image is capturedwith this system, each point in the electronic image includes amonochrome value and one color value, and the color image must have themissing colors at each pixel location interpolated from the nearbycolors. Although this system improves the light sensitivity over asingle conventional image sensor, the overall complexity, size, and costof the system is greater due to the need for two sensors and a beamsplitter. Furthermore, the beam splitter reduces the amount of light toeach image sensor by ½, limiting the improvement in photographic speed.

In addition to the linear image sensor mentioned above, there are knownin the art image sensors with two-dimensional arrays of pixels where thepixels include pixels that do not have color filters applied to them.For example, see Sato et al. in U.S. Pat. No. 4,390,895, Yamagami et al.in U.S. Pat. No. 5,323,233, and Gindele et al. in U.S. Pat. No.6,476,865. In each of the cited patents, the sensitivity of theunfiltered or monochrome pixels is significantly higher than the colorpixels, requiring the application of gain to the color pixels in orderto match the color and monochrome signals from the pixel array.Increasing gain increases noise as well as signal, causing degradationin the overall signal to noise ratio of the resulting image. Frame in USPatent Publication 2003/0210332 discloses a pixel array with most of thepixels being unfiltered, but the color pixels suffer from the samesensitivity deficit as mentioned above.

A need exists to provide high-resolution color images that areeffectively processed when captured in low light situations.

SUMMARY OF THE INVENTION

The present invention is directed to an improved method for processingdigital images having color and panchromatic pixels. Either linear ortwo-dimensional image array sensors that produce digital images havingcolor and panchromatic pixels can capture these pixels.

Briefly summarized, according to one aspect of the present invention,the invention provides a method for forming a final digital color image,comprising:

a) capturing an image using an image sensor having panchromatic pixelsand color pixels corresponding to at least two color photoresponses;

b) providing from the captured image a digital panchromatic image and anintermediate digital color image; and

c) using the digital panchromatic image and the intermediate digitalcolor image to provide the final digital color image.

By forming a digital image into a digital panchromatic image and anintermediate digital color image, an improved final digital color imageis provided. The panchromatic image can have a high-resolution and theintermediate digital color image can have a lower resolution. Theresulting final image has improved quality when the image was capturedunder varying lighting conditions.

Processing digital images having color and panchromatic pixels inaccordance with the present invention is particularly suitable fordigital images captured in low level lighting conditions, where such lowlevel lighting conditions are the result of low scene lighting, shortexposure time, small aperture, or other restriction on light reachingthe sensor. Said processing has broad application and numerous types ofimage capture devices can effectively use this processing.

These and other aspects, objects, features and advantages of the presentinvention will be more clearly understood and appreciated from a reviewof the following detailed description of the preferred embodiments andappended claims, and by reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a conventional digital still camera systemthat can employ a conventional sensor and processing methods or thesensor and processing methods of the current invention;

FIG. 2 (prior art) is conventional Bayer color filter array patternshowing a minimal repeating unit and a non-minimal repeating unit;

FIG. 3 provides representative spectral quantum efficiency curves forred, green, and blue pixels, as well as a wider spectrum panchromaticquantum efficiency, all multiplied by the transmission characteristicsof an infrared cut filter;

FIGS. 4A-D provides minimal repeating units for several variations of acolor filter array pattern of the present invention that has colorpixels with the same color photo response arranged in rows or columns;

FIG. 5 shows the cell structure of the minimal repeating unit from FIG.4A;

FIG. 6A is the interpolated panchromatic image for FIG. 4A;

FIG. 6B is the low-resolution color image corresponding to the cells inFIG. 4A and FIG. 5;

FIGS. 7A-C shows several ways of combining the pixels of FIG. 4A;

FIGS. 8A-D shows the color filter array pattern of FIG. 4A with colorpixels that have alternative color photoresponse characteristics,including four color alternatives as well as a cyan, magenta, and yellowalternatives;

FIG. 9 provides a minimal repeating unit for an alternative color filterarray of the present invention in which the panchromatic pixels arearranged in diagonal lines;

FIGS. 10A-B provides minimal repeating units for two variations of analternative color filter array of the present invention in which thepanchromatic pixels form a grid into which the color pixels areembedded;

FIGS. 11A-D provides minimal repeating units and tiling arrangements fortwo variations of an alternative color filter array of the presentinvention in which there are two colors per cell;

FIGS. 12A-B provides minimal repeating units for two variations of analternative color filter array of the present invention in which thereare two colors per cell and the panchromatic pixels are arranged indiagonal lines;

FIGS. 13A-C provides variations of FIG. 4A in which the minimalrepeating unit is smaller than eight by eight pixels;

FIGS. 14A-B provides minimal repeating units for two variations of analternative color filter array of the present invention in which theminimal repeating unit is six by six pixels;

FIGS. 15A-B provides minimal repeating units for two variations of analternative color filter array of the present invention in which theminimal repeating unit is four by four pixels;

FIG. 16 is the minimal repeating unit of FIG. 4A with subscripts forindividual pixels within the minimal repeating unit;

FIGS. 17A-E shows the panchromatic pixels and the color pixels of onecell of FIG. 16, and various ways in which the color pixels arecombined;

FIG. 18 is a process diagram of the present invention showing the methodof processing the color and panchromatic pixel data from a sensor of thepresent invention; and

FIGS. 19A-D illustrates methods of the present invention forinterpolating missing colors in the low-resolution partial color imageof FIG. 18.

DETAILED DESCRIPTION OF THE INVENTION

Because digital cameras employing imaging devices and related circuitryfor signal capture and correction and for exposure control are wellknown, the present description will be directed in particular toelements forming part of, or cooperating more directly with, method andapparatus in accordance with the present invention. Elements notspecifically shown or described herein are selected from those known inthe art. Certain aspects of the embodiments to be described are providedin software. Given the system as shown and described according to theinvention in the following materials, software not specifically shown,described or suggested herein that is useful for implementation of theinvention is conventional and within the ordinary skill in such arts.

Turning now to FIG. 1, a block diagram of an image capture device shownas a digital camera embodying the present invention is shown. Although adigital camera will now be explained, the present invention is clearlyapplicable to other types of image capture devices. In the disclosedcamera, light 10 from the subject scene is input to an imaging stage 11,where the light is focused by lens 12 to form an image on solid stateimage sensor 20. Image sensor 20 converts the incident light to anelectrical signal for each picture element (pixel). The image sensor 20of the preferred embodiment is a charge coupled device (CCD) type or anactive pixel sensor (APS) type (APS devices are often referred to asCMOS sensors because of the ability to fabricate them in a ComplementaryMetal Oxide Semiconductor process), Other types of image sensors havingtwo-dimensional array of pixels are used provided that they employ thepatterns of the present invention. The present invention also makes useof an image sensor 20 having a two-dimensional array of color andpanchromatic pixels as will become clear later in this specificationafter FIG. 1 is described. Examples of the patterns of color andpanchromatic pixels of the present invention that are used with theimage sensor 20 are seen in FIGS. 4A-D, FIGS. 8A-D, FIG. 9, FIGS. 10A-B,FIG. 11A, FIG. 11C, FIGS. 13A-C, FIGS. 14A-B, and FIGS. 15A-B, althoughother patterns are used within the spirit of the present invention.

The amount of light reaching the sensor 20 is regulated by an iris block14 that varies the aperture and the neutral density (ND) filter block 13that includes one or more ND filters interposed in the optical path.Also regulating the overall light level is the time that the shutterblock 18 is open. The exposure controller block 40 responds to theamount of light available in the scene as metered by the brightnesssensor block 16 and controls all three of these regulating functions.

This description of a particular camera configuration will be familiarto one skilled in the art, and it will be obvious that many variationsand additional features are present. For example, an autofocus system isadded, or the lens are detachable and interchangeable. It will beunderstood that the present invention is applied to any type of digitalcamera, where similar functionality is provided by alternativecomponents. For example, the digital camera is a relatively simple pointand shoot digital camera, where the shutter 18 is a relatively simplemovable blade shutter, or the like, instead of the more complicatedfocal plane arrangement. The present invention can also be practiced onimaging components included in non-camera devices such as mobile phonesand automotive vehicles.

The analog signal from image sensor 20 is processed by analog signalprocessor 22 and applied to analog to digital (A/D) converter 24. Timinggenerator 26 produces various clocking signals to select rows and pixelsand synchronizes the operation of analog signal processor 22 and A/Dconverter 24. The image sensor stage 28 includes the image sensor 20,the analog signal processor 22, the A/D converter 24, and the timinggenerator 26. The components of image sensor stage 28 is separatelyfabricated integrated circuits, or they are fabricated as a singleintegrated circuit as is commonly done with CMOS image sensors. Theresulting stream of digital pixel values from A/D converter 24 is storedin memory 32 associated with digital signal processor (DSP) 36.

Digital signal processor 36 is one of three processors or controllers inthis embodiment, in addition to system controller 50 and exposurecontroller 40. Although this partitioning of camera functional controlamong multiple controllers and processors is typical, these controllersor processors are combined in various ways without affecting thefunctional operation of the camera and the application of the presentinvention. These controllers or processors can comprise one or moredigital signal processor devices, microcontrollers, programmable logicdevices, or other digital logic circuits. Although a combination of suchcontrollers or processors has been described, it should be apparent thatone controller or processor is designated to perform all of the neededfunctions. All of these variations can perform the same function andfall within the scope of this invention, and the term “processing stage”will be used as needed to encompass all of this functionality within onephrase, for example, as in processing stage 38 in FIG. 1.

In the illustrated embodiment, DSP 36 manipulates the digital image datain its memory 32 according to a software program permanently stored inprogram memory 54 and copied to memory 32 for execution during imagecapture. DSP 36 executes the software necessary for practicing imageprocessing shown in FIG. 18. Memory 32 includes of any type of randomaccess memory, such as SDRAM. A bus 30 comprising a pathway for addressand data signals connects DSP 36 to its related memory 32, A/D converter24 and other related devices.

System controller 50 controls the overall operation of the camera basedon a software program stored in program memory 54, which can includeFlash EEPROM or other nonvolatile memory. This memory can also be usedto store image sensor calibration data, user setting selections andother data which must be preserved when the camera is turned off. Systemcontroller 50 controls the sequence of image capture by directingexposure controller 40 to operate the lens 12, ND filter 13, iris 14,and shutter 18 as previously described, directing the timing generator26 to operate the image sensor 20 and associated elements, and directingDSP 36 to process the captured image data. After an image is capturedand processed, the final image file stored in memory 32 is transferredto a host computer via interface 57, stored on a removable memory card64 or other storage device, and displayed for the user on image display88.

A bus 52 includes a pathway for address, data and control signals, andconnects system controller 50 to DSP 36, program memory 54, systemmemory 56, host interface 57, memory card interface 60 and other relateddevices. Host interface 57 provides a high speed connection to apersonal computer (PC) or other host computer for transfer of image datafor display, storage, manipulation or printing. This interface is anIEEE1394 or USB2.0 serial interface or any other suitable digitalinterface. Memory card 64 is typically a Compact Flash (CF) cardinserted into socket 62 and connected to the system controller 50 viamemory card interface 60. Other types of storage that are utilizedinclude without limitation PC-Cards, MultiMedia Cards (MMC), or SecureDigital (SD) cards.

Processed images are copied to a display buffer in system memory 56 andcontinuously read out via video encoder 80 to produce a video signal.This signal is output directly from the camera for display on anexternal monitor, or processed by display controller 82 and presented onimage display 88. This display is typically an active matrix colorliquid crystal display (LCD), although other types of displays are usedas well.

The user interface, including all or any combination of viewfinderdisplay 70, exposure display 72, status display 76 and image display 88,and user inputs 74, is controlled by a combination of software programsexecuted on exposure controller 40 and system controller 50. User inputs74 typically include some combination of buttons, rocker switches,joysticks, rotary dials or touchscreens. Exposure controller 40 operateslight metering, exposure mode, autofocus and other exposure functions.The system controller 50 manages the graphical user interface (GUI)presented on one or more of the displays, e.g., on image display 88. TheGUI typically includes menus for making various option selections andreview modes for examining captured images.

Exposure controller 40 accepts user inputs selecting exposure mode, lensaperture, exposure time (shutter speed), and exposure index or ISO speedrating and directs the lens and shutter accordingly for subsequentcaptures. Brightness sensor 16 is employed to measure the brightness ofthe scene and provide an exposure meter function for the user to referto when manually setting the ISO speed rating, aperture and shutterspeed. In this case, as the user changes one or more settings, the lightmeter indicator presented on viewfinder display 70 tells the user towhat degree the image will be over or underexposed. In an automaticexposure mode, the user changes one setting and the exposure controller40 automatically alters another setting to maintain correct exposure,e.g., for a given ISO speed rating when the user reduces the lensaperture the exposure controller 40 automatically increases the exposuretime to maintain the same overall exposure.

The ISO speed rating is an important attribute of a digital stillcamera. The exposure time, the lens aperture, the lens transmittance,the level and spectral distribution of the scene illumination, and thescene reflectance determine the exposure level of a digital stillcamera. When an image from a digital still camera is obtained using aninsufficient exposure, proper tone reproduction can generally bemaintained by increasing the electronic or digital gain, but the imagewill contain an unacceptable amount of noise. As the exposure isincreased, the gain is decreased, and therefore the image noise cannormally be reduced to an acceptable level. If the exposure is increasedexcessively, the resulting signal in bright areas of the image canexceed the maximum signal level capacity of the image sensor or camerasignal processing. This can cause image highlights to be clipped to forma uniformly bright area, or to bloom into surrounding areas of theimage. It is important to guide the user in setting proper exposures. AnISO speed rating is intended to serve as such a guide. In order to beeasily understood by photographers, the ISO speed rating for a digitalstill camera should directly relate to the ISO speed rating forphotographic film cameras. For example, if a digital still camera has anISO speed rating of ISO 200, then the same exposure time and apertureshould be appropriate for an ISO 200 rated film/process system.

The ISO speed ratings are intended to harmonize with film ISO speedratings. However, there are differences between electronic andfilm-based imaging systems that preclude exact equivalency. Digitalstill cameras can include variable gain, and can provide digitalprocessing after the image data has been captured, enabling tonereproduction to be achieved over a range of camera exposures. It istherefore possible for digital still cameras to have a range of speedratings. This range is defined as the ISO speed latitude. To preventconfusion, a single value is designated as the inherent ISO speedrating, with the ISO speed latitude upper and lower limits indicatingthe speed range, that is, a range including effective speed ratings thatdiffer from the inherent ISO speed rating. With this in mind, theinherent ISO speed is a numerical value calculated from the exposureprovided at the focal plane of a digital still camera to producespecified camera output signal characteristics. The inherent speed isusually the exposure index value that produces peak image quality for agiven camera system for normal scenes, where the exposure index is anumerical value that is inversely proportional to the exposure providedto the image sensor.

The foregoing description of a digital camera will be familiar to oneskilled in the art. It will be obvious that there are many variations ofthis embodiment that are possible and is selected to reduce the cost,add features or improve the performance of the camera. The followingdescription will disclose in detail the operation of this camera forcapturing images according to the present invention. Although thisdescription is with reference to a digital camera, it will be understoodthat the present invention applies for use with any type of imagecapture device having an image sensor with color and panchromaticpixels.

The image sensor 20 shown in FIG. 1 typically includes a two-dimensionalarray of light sensitive pixels fabricated on a silicon substrate thatprovide a way of converting incoming light at each pixel into anelectrical signal that is measured. As the sensor is exposed to light,free electrons are generated and captured within the electronicstructure at each pixel. Capturing these free electrons for some periodof time and then measuring the number of electrons captured, ormeasuring the rate at which free electrons are generated can measure thelight level at each pixel. In the former case, accumulated charge isshifted out of the array of pixels to a charge to voltage measurementcircuit as in a charge coupled device (CCD), or the area close to eachpixel can contain elements of a charge to voltage measurement circuit asin an active pixel sensor (APS or CMOS sensor).

Whenever general reference is made to an image sensor in the followingdescription, it is understood to be representative of the image sensor20 from FIG. 1. It is further understood that all examples and theirequivalents of image sensor architectures and pixel patterns of thepresent invention disclosed in this specification is used for imagesensor 20.

In the context of an image sensor, a pixel (a contraction of “pictureelement”) refers to a discrete light sensing area and charge shifting orcharge measurement circuitry associated with the light sensing area. Inthe context of a digital color image, the term pixel commonly refers toa particular location in the image having associated color values.

In order to produce a color image, the array of pixels in an imagesensor typically has a pattern of color filters placed over them. FIG. 2shows a pattern of red, green, and blue color filters that is commonlyused. This particular pattern is commonly known as a Bayer color filterarray (CFA) after its inventor Bryce Bayer as disclosed in U.S. Pat. No.3,971,065. This pattern is effectively used in image sensors having atwo-dimensional array of color pixels. As a result, each pixel has aparticular color photoresponse that, in this case, is a predominantsensitivity to red, green or blue light. Another useful variety of colorphotoresponses is a predominant sensitivity to magenta, yellow, or cyanlight. In each case, the particular color photoresponse has highsensitivity to certain portions of the visible spectrum, whilesimultaneously having low sensitivity to other portions of the visiblespectrum. The term color pixel will refer to a pixel having a colorphotoresponse.

The set of color photoresponses selected for use in a sensor usually hasthree colors, as shown in the Bayer CFA, but it can also include four ormore. As used herein, a panchromatic photoresponse refers to aphotoresponse having a wider spectral sensitivity than those spectralsensitivities represented in the selected set of color photoresponses. Apanchromatic photosensitivity can have high sensitivity across theentire visible spectrum. The term panchromatic pixel will refer to apixel having a panchromatic photoresponse. Although the panchromaticpixels generally have a wider spectral sensitivity than the set of colorphotoresponses, each panchromatic pixel can have an associated filter.Such filter is either a neutral density filter or a color filter.

When a pattern of color and panchromatic pixels is on the face of animage sensor, each such pattern has a repeating unit that is acontiguous subarray of pixels that acts as a basic building block. Byjuxtaposing multiple copies of the repeating unit, the entire sensorpattern is produced. The juxtaposition of the multiple copies ofrepeating units are done in diagonal directions as well as in thehorizontal and vertical directions.

A minimal repeating unit is a repeating unit such that no otherrepeating unit has fewer pixels. For example, the CFA in FIG. 2 includesa minimal repeating unit that is two pixels by two pixels as shown bypixel block 100 in FIG. 2. Multiple copies of this minimal repeatingunit is tiled to cover the entire array of pixels in an image sensor.The minimal repeating unit is shown with a green pixel in the upperright corner, but three alternative minimal repeating units can easilybe discerned by moving the heavy outlined area one pixel to the right,one pixel down, or one pixel diagonally to the right and down. Althoughpixel block 102 is a repeating unit, it is not a minimal repeating unitbecause pixel block 100 is a repeating unit and block 100 has fewerpixels than block 102.

An image captured using an image sensor having a two-dimensional arraywith the CFA of FIG. 2 has only one color value at each pixel. In orderto produce a full color image, there are a number of techniques forinferring or interpolating the missing colors at each pixel. These CFAinterpolation techniques are well known in the art and reference is madeto the following patents: U.S. Pat. Nos. 5,506,619, 5,629,734, and5,652,621.

FIG. 3 shows the relative spectral sensitivities of the pixels with red,green, and blue color filters in a typical camera application. TheX-axis in FIG. 3 represents light wavelength in nanometers, and theY-axis represents efficiency. In FIG. 3, curve 110 represents thespectral transmission characteristic of a typical filter used to blockinfrared and ultraviolet light from reaching the image sensor. Such afilter is needed because the color filters used for image sensorstypically do not block infrared light, hence the pixels are unable todistinguish between infrared light and light that is within thepassbands of their associated color filters. The infrared blockingcharacteristic shown by curve 110 prevents infrared light fromcorrupting the visible light signal. The spectral quantum efficiency,i.e. the proportion of incident photons that are captured and convertedinto a measurable electrical signal, for a typical silicon sensor withred, green, and blue filters applied is multiplied by the spectraltransmission characteristic of the infrared blocking filter representedby curve 110 to produce the combined system quantum efficienciesrepresented by curve 114 for red, curve 116 for green, and curve 118 forblue. It is understood from these curves that each color photoresponseis sensitive to only a portion of the visible spectrum. By contrast, thephotoresponse of the same silicon sensor that does not have colorfilters applied (but including the infrared blocking filtercharacteristic) is shown by curve 112; this is an example of apanchromatic photoresponse. By comparing the color photoresponse curves114, 116, and 118 to the panchromatic photoresponse curve 112, it isclear that the panchromatic photoresponse is three to four times moresensitive to wide spectrum light than any of the color photoresponses.

The greater panchromatic sensitivity shown in FIG. 3 permits improvingthe overall sensitivity of an image sensor by intermixing pixels thatinclude color filters with pixels that do not include color filters.However, the color filter pixels will be significantly less sensitivethan the panchromatic pixels. In this situation, if the panchromaticpixels are properly exposed to light such that the range of lightintensities from a scene cover the full measurement range of thepanchromatic pixels, then the color pixels will be significantlyunderexposed. Hence, it is advantageous to adjust the sensitivity of thecolor filter pixels so that they have roughly the same sensitivity asthe panchromatic pixels. The sensitivity of the color pixels areincreased, for example, by increasing the size of the color pixelsrelative to the panchromatic pixels, with an associated reductioninspatial pixels.

FIG. 4A represents a two-dimensional array of pixels having two groups.Pixels from the first group of pixels have a narrower spectralphotoresponse than pixels from the second group of pixels. The firstgroup of pixels includes individual pixels that relate to at least twodifferent spectral photoresponses corresponding to at least two colorfilters. These two groups of pixels are intermixed to improve theoverall sensitivity of the sensor. As will become clearer in thisspecification, the placement of the first and second groups of pixelsdefines a pattern that has a minimal repeating unit including at leasttwelve pixels. The minimal repeating unit includes first and secondgroups of pixels arranged to permit the reproduction of a captured colorimage under different lighting conditions.

The complete pattern shown in FIG. 4A represents a minimal repeatingunit that is tiled to cover an entire array of pixels. As with FIG. 2,there are several other minimal repeating units that are used todescribe this overall arrangement of color and panchromatic pixels, butthey are all essentially equivalent in their characteristics and each isa subarray of pixels, the subarray being eight pixels by eight pixels inextent. An important feature of this pattern is alternating rows ofpanchromatic and color pixels with the color rows having pixels with thesame color photoresponse grouped together. The groups of pixels with thesame photoresponse along with some of their neighboring panchromaticpixels are considered to form four cells that make up the minimalrepeating unit, a cell being a contiguous subarray of pixels havingfewer pixels than a minimal repeating unit.

These four cells, delineated by heavy lines in FIG. 4A and shown ascells 120, 122, 124, and 126 in FIG. 5, enclose four groups offour-by-four pixels each, with 120 representing the upper left cell, 122representing the upper right cell, 124 representing the lower left cell,and 126 representing the lower right cell. Each of the four cellsincludes eight panchromatic pixels and eight color pixels of the samecolor photoresponse. The color pixels in a cell is combined to representthe color for that entire cell. Hence, cell 120 in FIG. 5 is consideredto be a green cell, cell 122 is considered to be a red cell, and so on.Each cell includes at least two pixels of the same color, therebyallowing pixels of the same color to be combined to overcome thedifference in photosensitivity between the color pixels and thepanchromatic pixels.

In the case of a minimal repeating unit with four non-overlapping cells,with each cell having two pixels of the same color and two panchromaticpixels, it is clear that the minimal repeating unit includes sixteenpixels. In the case of a minimal repeating unit with threenon-overlapping cells, with each cell having two pixels of the samecolor and two panchromatic pixels, it is clear that the minimalrepeating unit includes twelve pixels.

In accordance with the present invention, the minimal repeating unit ofFIG. 4A, when considered in light of the cell structure identified inFIG. 5, can represent the combination of a high-resolution panchromaticimage and a low-resolution Bayer pattern color image arranged to permitthe reproduction of a captured color image under different lightingconditions. The individual elements of the Bayer pattern image representthe combination of the color pixels in the corresponding cells. Thefirst group of pixels defines a low-resolution color filter array imageand the second group of pixels defines a high-resolution panchromaticimage. See FIG. 6A and FIG. 6B. FIG. 6A represents the high-resolutionpanchromatic image corresponding to FIG. 4A, including both thepanchromatic pixels P from FIG. 4A as well as interpolated panchromaticpixels P′; and FIG. 6B represents the low-resolution Bayer pattern colorimage, with R′, G′, and B′ representing for each of the cells outlinedin FIG. 5 the cell color associated with the combined color pixels inthe cell.

In the following discussion, all cells in FIGS. 4B-D, 8A-D, 9, 10A-B,11A, 11C, 12A-B, 13A-C, 14A-B, and 15A-B are delineated by heavy lines,as they were in FIG. 4A.

In addition to alternative minimal repeating units of FIG. 4A, each cellof the pattern is rotated 90 degrees to produce the pattern shown inFIG. 4B. This is substantially the same pattern, but it places thehighest panchromatic sampling frequency in the vertical directioninstead of the horizontal direction. The choice to use FIG. 4A or FIG.4B depends on whether or not it is desired to have higher panchromaticspatial sampling in either the horizontal or vertical directionsrespectively. However, it is clear that the resulting cells that make upthe minimal repeating unit in both patterns produce the samelow-resolution color image for both patterns. Hence, FIG. 4A and FIG. 4Bare equivalent from a color perspective. In general, FIG. 4A and FIG. 4Bare examples of practicing the present invention with the panchromaticpixels arranged linearly in either rows or columns. Furthermore, FIG. 4Ahas single rows of panchromatic pixels with each row separated from aneighboring row of panchromatic pixels by a row of color pixels; FIG. 4Bhas the same characteristic in the column direction.

FIG. 4C represents yet another alternative minimal repeating unit toFIG. 4A with essentially the same cell color characteristics. However,FIG. 4C shows the panchromatic and color rows staggered on a cell bycell basis. This can improve the vertical panchromatic resolution. Yetanother alternative minimal repeating unit to FIG. 4A is represented inFIG. 4D, wherein the panchromatic and color rows are staggered by columnpairs. This also has the potential of improving the verticalpanchromatic resolution. A characteristic of all of the minimalrepeating units of FIGS. 4A-D is that groups of two or more same colorpixels are arranged side by side in either rows or columns.

FIGS. 4A-D all have the same color structure with the cells thatconstitute the minimal repeating unit expressing a low-resolution Bayerpattern. It can therefore be seen that a variety of arrangements ofpanchromatic pixels and grouped color pixels are constructed within thespirit of the present invention.

In order to increase the color photosensitivity to overcome thedisparity between the panchromatic photosensitivity and the colorphotosensitivity, the color pixels within each cell is combined invarious ways. For example, the charge from same colored pixels arecombined or binned in a CCD image sensor or in types of active pixelsensors that permit binning. Alternatively, the voltages correspondingto the measured amounts of charge in same colored pixels are averaged,for example by connecting in parallel capacitors that are charged tothese voltages. In yet another approach, the digital representations ofthe light levels at same colored pixels are summed or averaged.Combining or binning charge from two pixels doubles the signal level,while the noise associated with sampling and reading out the combinedsignal remains the same, thereby increasing the signal to noise ratio bya factor of two, representing a corresponding two times increase in thephotosensitivity of the combined pixels. In the case of summing thedigital representations of the light levels from two pixels, theresulting signal increases by a factor of two, but the correspondingnoise levels from reading the two pixels combine in quadrature, therebyincreasing the noise by the square root of two; the resulting signal tonoise ratio of the combined pixels therefore increases by the squareroot of two over the uncombined signals. A similar analysis applies tovoltage or digital averaging.

The previously mentioned approaches for combining signals from samecolored pixels within a cell is used singly or in combinations. Forexample, by vertically combining the charge from same colored pixels inFIG. 4A in groups of two to produce the combined pixels with combinedsignals R′, G′, and B′ shown in FIG. 7A. In this case, each R′, G′, andB′ has twice the sensitivity of the uncombined pixels. Alternatively,horizontally combining the measured values, (either voltage or digital)from same colored pixels in FIG. 4A in groups of four produces thecombined pixels with combined signals R′, G′, and B′ shown in FIG. 7B.In this case, since the signal increases by a factor of four but thenoise increases by 2, each R′, G′, and B′ has twice the sensitivity ofthe uncombined pixels. In another alternative combination scheme,vertically combining the charge from same colored pixels in groups oftwo as in FIG. 7A, and horizontally summing or averaging the measuredvalues of the combined pixels of FIG. 7A in groups of four produces thefinal combined color pixels of FIG. 7C, with R″, G″, and B″ representingthe final combinations of same colored pixels. In this combinationarrangement, the final combined color pixels of FIG. 7C each have fourtimes the sensitivity of the uncombined pixels. Some sensorarchitectures, notably certain CCD arrangements, can permit the chargefrom all eight same colored pixels within each cell to be combined inthe fashion of FIG. 7C, leading to an eightfold increase in sensitivityfor the combined color pixels.

From the foregoing, it will now be understood that there are severaldegrees of freedom in combining color pixels for the purpose ofadjusting the photosensitivity of the color pixels. Well known combiningschemes will suggest themselves to one skilled in the art and is basedon scene content, scene illuminant, overall light level, or othercriteria. Furthermore, the combining scheme is selected to deliberatelypermit the combined pixels to have either less sensitivity or moresensitivity than the panchromatic pixels.

To this point the image sensor has been described as employing red,green, and blue filters. The present invention is practiced withalternative filter selections. Image sensors employing cyan, magenta,and yellow sensors are well known in the art, and the present inventionis practiced with cyan, magenta, and yellow color filters. FIG. 8A showsthe cyan, magenta, and yellow equivalent of FIG. 4A, with C representingcyan pixels, M representing magenta pixels, and Y representing yellowpixels. The present invention is also usable with pixels having morethan three color photoresponses.

FIG. 8B shows a minimal repeating unit of the present invention thatincludes cyan pixels (represented by C), magenta pixels (represented byM), yellow pixels (represented by Y), and green pixels (represented byG). This retains the overall cell arrangement of the minimal repeatingunit shown in FIG. 5, but includes four different colored pixels andtherefore four different colored corresponding cells. FIG. 8C shows yetanother alternative four color arrangement including red pixels(represented by R), blue pixels (represented by B), green pixels withone color photoresponse (represented by G), and alternative green pixelswith a different color photoresponse (represented by E). FIG. 8D showsyet another alternative four color arrangement, wherein one of the greencells of FIG. 4A is replaced by a yellow cell, with the yellow pixelsrepresented by Y.

The present invention is practiced with fewer than three colors inaddition to the panchromatic pixels. For example, a minimal repeatingunit with cells corresponding to the colors red and blue is suitable foruse.

Many alternatives to FIG. 4A are practiced within the spirit of thepresent invention. For example, FIG. 9 represents an alternative minimalrepeating unit of the present invention with the same cell structure asFIG. 4A but with a checkerboard pattern of panchromatic pixels. Thispattern provides uniform panchromatic sampling of the image, overcomingthe vertical panchromatic sampling deficit of FIGS. 4A, 4C, and 4D. FIG.9 is characterized as an example of practicing the present invention byarranging the panchromatic pixels in diagonal lines. FIG. 9 is furthercharacterized as having single diagonal lines of panchromatic pixelswith each diagonal line separated from a neighboring diagonal line ofpanchromatic pixels by a diagonal line of color pixels. Yet anothercharacteristic of FIG. 9 is that groups two or more of same color pixelsare arranged side by side in diagonal lines.

The patterns presented so far have had equal numbers of panchromatic andcolor pixels. The present invention is not limited to this arrangementas there are more panchromatic pixels than color pixels. FIG. 10A showsyet another embodiment of the present invention wherein color pixels areembedded within a grid pattern of panchromatic pixels. This patternprovides very good panchromatic spatial sampling while expressing thesame color cell arrangement as FIGS. 4A and 9. FIG. 10B provides anexample of a four color embodiment of the panchromatic grid pattern. Ingeneral, the minimal repeating unit of FIG. 10 is characterized asseparating each color pixel from a neighboring color pixel by one ormore panchromatic pixels.

For a given pixel pattern, a minimal repeating unit has been previouslydefined as a repeating unit such that no other repeating unit has fewerpixels. In the same sense, the sizes of repeating units from differentpixel patterns are compared according to the total number of pixels inthe repeating unit. As an example, a four pixel by eight pixel repeatingunit from one pixel pattern is smaller than a six pixel by six pixelrepeating unit from another pixel pattern because the total number ofpixels (4×8=32) in the first repeating unit is smaller than the totalnumber of pixels (6×6=36) in the second repeating unit. As a furtherexample, a repeating unit that is smaller than a repeating unit havingeight pixels by eight pixels contains fewer than 64 total pixels.

All the patterns presented so far have exhibited a cell structurewherein each cell contains a single color in addition to panchromaticpixels. Furthermore, all the patterns presented so far have exhibited aminimal repeating unit that is eight by eight pixels in extent. Aminimal repeating unit can also be used that has cells with more thanone color in each cell; also, a minimal repeating unit is defined thatis less than eight pixels by eight pixels in extent. For example, theminimal repeating unit of FIG. 11A has two cells with each cellincluding two colors: blue and green (represented by B and Grespectively) in the left cell, and red and green (represented by R andG respectively) in the right cell. In FIG. 11A the cells contain twocolors, and these colors are arranged to facilitate combining samecolors for the purpose of improving color sensitivity. FIG. 11B showshow the minimal repeating unit of FIG. 11A is tiled in order to staggerthe red and blue colors. FIG. 11C provides a minimal repeating unitemploying four colors and two colors per cell. FIG. 11D shows how theminimal repeating unit of FIG. 11C is tiled in order to stagger the redand blue colors. In FIG. 11D the coarse color pattern is characterizedas a checkerboard of two different color photoresponses in the greenrange (represented by G and E) interleaved with a checkerboard of redand blue (represented by R and B, respectively). FIG. 12A provides apanchromatic checkerboard version of FIG. 11A, and FIG. 12B provides apanchromatic checkerboard version of FIG. 11C. In general, the minimalrepeating units of FIGS. 11A and 11C are characterized as separatingeach color pixel from a neighboring color pixel in rows and columns by adissimilar pixel, either a different color pixel or a panchromaticpixel.

The minimal repeating units described so far have been eight by eight ortwo by eight pixels in extent. However, the minimal repeating unit issmaller. For example, FIG. 13A is analogous to FIG. 4A, but with eachcolor cell being 3 pixels wide by 4 pixels high and with the overallminimal repeating unit being 6 pixels wide by 8 pixels high. FIG. 13Beliminates two of the color pixel rows from FIG. 13A, thereby producingcells that are 3 pixels by 3 pixels and a minimal repeating unit that is6 pixels by 6 pixels. FIG. 13C goes further by eliminating two of thepanchromatic rows, thereby producing cells that are 3 pixels wide by 2pixels high (with each cell containing 3 panchromatic pixels and 3 colorpixels) and a minimal repeating unit that is 6 pixels wide by 4 pixelstall. The patterns shown in FIGS. 13A through 13C are particularlyusable if the scheme for combining colors within each cell requires lessthan the numbers of pixels shown in FIG. 4A and other patterns.

FIG. 14A shows yet another minimal repeating unit. The minimal repeatingunit in FIG. 14A is six pixels by six pixels, with each cell including a4 pixel diamond pattern of a single color with the remaining 5 pixelsbeing panchromatic pixels. The panchromatic spatial sampling patternshown in FIG. 14A is somewhat irregular, suggesting the pattern of FIG.14B with a panchromatic checkerboard and the remaining pixels in eachthree pixel by three pixel cell occupied by a single color.

FIG. 15A shows a minimal repeating unit that is four by four pixels andincludes four two by two pixel cells. Note that each cell includes twopanchromatic pixels and two same color pixels. The invention requiresthe placement of two same color pixels in each of the two by two cellsin order to facilitate combining the color pixels within each cell. FIG.15B is similar to FIG. 15A but employs a panchromatic checkerboardpattern.

Turning now to FIG. 16, the minimal repeating unit of FIG. 5 is shownsubdivided into four cells, a cell being a contiguous subarray of pixelshaving fewer pixels than a minimal repeating unit. The software neededto provide the following processing is included in DSP 36 of FIG. 1.Cells 220, 224, 226, and 228 are examples of cells wherein these cellscontain pixels having green, red, blue and green photoresponses,respectively. In this example, cell 220 contains both panchromaticpixels and green pixels, the green pixels being identified as pixelgroup 222. The eventual goal is to produce a single green signal forcell 220 by combining the eight green signals from the green pixels inpixel group 222. Depending on the image sensor's mode of operation, asingle green signal is produced by combining all eight green signals inthe analog domain (e.g. by charge binning), or multiple green signalsare produce by combining smaller groups of pixels taken from pixel group222. The panchromatic pixels of cell 220 are shown in FIG. 17A. In thefollowing examples, all eight signals from these panchromatic pixels areindividually digitized. The green pixels of cell 220 are shown in FIGS.17B-17E wherein they are grouped together according to how their signalsare combined in the analog domain. FIG. 17B depicts the case in whichall eight green pixels are combined to produce a single green signal forcell 220 (FIG. 16). The sensor can produce two green signals, forexample, by first combining the signals from pixels G21, G22, G23, andG24, and then combining the signals from pixels G41, G42, G43, and G44,as shown in FIG. 17C. Two signals are produced in other ways as well.The sensor can first combine signals from pixels G21, G22, G41, and G42,and then combine signals from pixels G23, G24, G43, and G44, as shown inFIG. 17D. The sensor can also produce four green signals for cell 220 bycombining four pairs of signals, for example, combining pixels G21 withG22, then combining G23 with G24, then combining G41 with G42, andfinally combining G43 with G44, as shown in FIG. 17E. It is clear thatthere are many additional ways to combine pairs of green signals withincell 220 (FIG. 16). If the sensor does no combining at all, then alleight green signals are reported individually for cell 220. Thus, in thecase of cell 220, the sensor can produce one, two, four or eight greenvalues for cell 220, and produce them in different ways, depending onits mode of operation.

For cells 224, 226, and 228 (FIG. 16), similar color signals areproduced by the sensor depending on its mode of operation. The colorsignals for cells 224, 226, and 228 are red, blue, and green,respectively.

Returning to the case of cell 220, regardless of how many signals aredigitized for this cell, the image processing algorithm of the presentinvention further combines the digitized green values to produce asingle green value for the cell. One way that a single green value isobtained is by averaging all the digitized green values produced forcell 220. In the event that a cell contains color pixels of differingphotoresponses, all the color data within the cell is similarly combinedso that there is a single value for each color photoresponse representedwithin the cell.

It is important to distinguish between the color values pertaining topixels in the original sensor that captured the raw image data, andcolor values pertaining to cells within the original sensor. Both typesof color values are used to produce color images, but the resultingcolor images are of different resolution. An image having pixel valuesassociated with pixels in the original sensor is referred to as ahigh-resolution image, and an image having pixel values associated withcells within the original sensor is referred to as a low-resolutionimage.

Turning now to FIG. 18, the digital signal processor block 36 (FIG. 1)is shown receiving captured raw image data from the data bus 30 (FIG.1). The raw image data is passed to both the Low-resolution PartialColor block 202 and the High-resolution Panchrome block 204. An exampleof a minimal repeating unit for an image sensor has already been shownin FIG. 5 and FIG. 16. In the case of cell 220 (FIG. 16), the capturedraw image data includes the panchromatic data that is produced by theindividual panchromatic pixels as shown in FIG. 17A. Also, for cell 220(FIG. 16), one or more green (color) values are also included, forexample, from the combinations shown in FIGS. 17B-E.

In the Low-resolution Partial Color block 202 (FIG. 18), a partial colorimage is produced from the captured raw image data, a partial colorimage being a color image wherein each pixel has at least one colorvalue and each pixel is also missing at least one color value. Dependingon the sensor's mode of operation, the captured raw data contains somenumber of color values produced by the color pixels within each cell.Within the Low-resolution Partial Color block 202, these color valuesare reduced to a single value for each color represented within thecell. For the cell 220 (FIG. 16), as an example, a single green colorvalue is produced. Likewise, for cells 224, 226 and 228, a single red,blue and green color value is produced, respectively.

The Low-resolution Partial Color block 202 processes each cell in asimilar manner resulting in an array of color values, one for each cell.Because the resulting image array based on cells rather than pixels inthe original sensor, it is four times smaller in each dimension than theoriginal captured raw image data array. Because the resulting array isbased on cells and because each pixel has some but not all color values,the resulting image is a low-resolution partial color image. At thispoint, the low-resolution partial color image is color balanced.

Looking now at the High-resolution Panchrome block 204, the same rawimage data is used as shown in FIG. 16, although the only thepanchromatic values will be used (FIG. 17A). This time the task is tointerpolate a complete high-resolution panchromatic image by estimatingpanchromatic values at those pixels not having panchromatic valuesalready. In the case of cell 220 (FIG. 16), panchromatic values must beestimated for the green pixels in pixel group 222 (FIG. 16). One simpleway to estimate the missing panchromatic values is to do verticalaveraging. Thus, for example, we can estimate the panchromatic value atpixel 22 as follows:P22=(P12+P32)/2An adaptive method can also be used. For example, one adaptive method isto compute three gradient values and take their absolute values:SCLAS=ABS(P31−P13)VCLAS=ABS(P32−P12)BCLAS=ABS(P33−P11)using the panchromatic values are shown in FIG. 17A. Likewise, threepredictor values are computed:SPRED=(P31+P13)/2VPRED=(P32+P12)/2BPRED=(P33+P11)/2

Then, set P22 equal to the predictor corresponding to the smallestclassifier value. In the case of a tie, set P22 equal to the average theindicated predictors. The panchromatic interpolation is continuedthroughout the image without regard to cell boundaries. When theprocessing of High-resolution Panchrome block 204 is done, the resultingdigital panchromatic image is the same size as the original captured rawimage, which makes it a high-resolution panchromatic image.

The Low-resolution Panchrome block 206 receives the high-resolutionpanchromatic image array produced by block 204 and generates alow-resolution panchromatic image array which is the same size as thelow-resolution partial color image produced by block 202. Eachlow-resolution panchromatic value is obtained by averaging the estimatedpanchromatic values, within a given cell, for those pixels having colorfilters. In the case of cell 220 (FIG. 16) the high-resolutionpanchromatic values, previously estimated for the green pixels in pixelgroup 222 (FIG. 16), are now averaged together to produce a singlelow-resolution panchromatic value for the cell. Likewise, a singlelow-resolution panchromatic value is computed for cell 224 usinghigh-resolution panchromatic values estimated at the pixels having redfilters. In this manner, each cell ends up with a single low-resolutionpanchromatic value.

The Low-resolution Color Difference block 208 receives thelow-resolution partial color image from block 202 and the low-resolutionpanchrome array from block 206. A low-resolution intermediate colorimage is then formed by color interpolating the low-resolution partialcolor image with guidance from the low-resolution panchrome image. Theexact nature of the color interpolation algorithm, to be discussed indetail later, depends on which pattern of pixel photoresponses was usedto capture the original raw image data.

After the low-resolution intermediate color image is formed it is colorcorrected. Once the low-resolution intermediate color image is colorcorrected, a low-resolution image of color differences are computed bysubtracting the low-resolution panchromatic image from each of thelow-resolution color planes individually. The High-resolution ColorDifference block 210 receives the low-resolution color difference imagefrom block 208 and, using bilinear interpolation, upsamples thelow-resolution color difference image to match the size of the originalraw image data. The result is a high-resolution color difference imagethat is the same size as the high-resolution panchromatic image producedby block 204.

The High-resolution Final Image block 212 receives the high-resolutioncolor difference image from block 210 and the high-resolutionpanchromatic image from block 204. A high-resolution final color imageis then formed by adding the high-resolution panchromatic image to eachof the high-resolution color difference planes. The resultinghigh-resolution final color image can then be further processed. Forexample, it is stored in the DSP Memory block 32 (FIG. 1) and thensharpened and compressed for storage on the Memory Card block 64 (FIG.1).

The sensor filter patterns shown in FIGS. 4A-D, 8A, 9, 10A, 13A-C, 14A-Band 15A-B have a minimal repeating unit such that the resultinglow-resolution partial color image, produced in block 202, exhibits therepeating Bayer pattern for color filters:

-   -   G R    -   B G        In addition to a single color value, given by the low-resolution        partial color image, every cell also has a panchromatic value        given by the low-resolution panchromatic image.

Considering the case in which the Bayer pattern is present in thelow-resolution partial color image, the task of color interpolationwithin the Low-resolution Color Differences block 208 (FIG. 18) can nowbe described in greater detail. Color interpolation begins byinterpolating the green values at pixels not already having greenvalues, shown as pixel 234 in FIG. 19A. The four neighboring pixels,shown as pixels 230, 232, 236, and 238, all have green values and theyalso all have panchromatic values. The center pixel 234 has apanchromatic value, but does not have a green value as indicated by thequestion marks.

The first step is to compute two classifier values, the first relatingto the horizontal direction, and the second to the vertical direction:HCLAS=ABS(P4−P2)+ABS(2*P3−P2−P4)VCLAS=ABS(P5−P1)+ABS(2*P3−P1−P5)Then, compute two predictor values, the first relating to the horizontaldirection, and the second to the vertical direction:HPRED=(G4+G2)/2+(2*P3−P2−P4)/2VPRED=(G5+G1)/2+(2*P3−P1−P5)/2Finally, letting THRESH be an empirically determined threshold value, wecan adaptively compute the missing value, G3, according to:

IF MAX(HCLAS, VCLAS) < THRESH G3 = (HPRED + VPRED)/2 ELSEIF VCLAS <HCLAS G3 = VPRED ELSE G3 = HPRED ENDThus, if both classifiers are smaller than the threshold value, anaverage of both predictor values is computed for G3. If not, then eitherHPRED or VPRED is used depending on which classifier HCLAS or VCLAS issmaller.

Once all the missing green values have been estimated, the missing redand blue values are interpolated. As shown in FIG. 19B, pixel 242 ismissing a red value but its two horizontal neighbors, pixels 240 and244, have red values R2 and R4 respectively. All three pixels have greenvalues. Under these conditions, an estimate for the red value (R3) forpixel 242 is computed as follows:R3=(R4+R2)/2+(2*G3−G2−G4)/2Missing blue values are computed in a similar way under similarconditions. At this point, the only pixels that still have missing redand blue values are those requiring vertical interpolation. As shown inFIG. 19C, pixel 252 is missing a red value and its two verticalneighbors, pixels 250 and 254, have red values R1 and R5 respectively.Under these conditions, an estimate for the red value (R3) for pixel 252is computed as follows:R3=(R5+R1)/2+(2*G3−G1−G5)/2Missing blue values are computed in a similar way under similarconditions. This completes the interpolation of the low-resolutionpartial color image and the result is a low-resolution intermediatecolor image. As described earlier, the low-resolution color differencescan now be computed by subtracting the low-resolution panchrome valuesfrom each color plane: red, green, and blue in the example justdiscussed.

Not all sensors produce low-resolution partial color images exhibiting arepeating Bayer pattern of color values. For example, the sensor patternshown in FIG. 11A determines that each cell receives two color values:either green and red, or green and blue. Consequently, in this case, thecolor interpolation task within the Low-resolution Color Differencesblock 208 (FIG. 18) estimates missing values of red or missing values ofblue for each pixel. Referring to FIG. 19D, a pixel 264 is shown havinga green value (G3) but not having a red value (R3). Four of theneighboring pixels 260, 262, 266, and 268 have green values and redvalues. The method for interpolating the red value for pixel 264 (FIG.19D) is similar to the method used to interpolate the green value forpixel 234 (FIG. 19A).

The first step is to compute two classifier values, the first relatingto the horizontal direction, and the second to the vertical direction:HCLAS=ABS(G4−G2)+ABS(2*G3−G2−G4)VCLAS=ABS(G5−G1)+ABS(2*G3−G1−G5)Then, compute two predictor values, the first relating to the horizontaldirection, and the second to the vertical direction:HPRED=(R4+R2)/2+(2*G3−G2−G4)/2VPRED=(R5+R1)/2+(2*G3−G1−G5)/2Finally, letting THRESH be an empirically determined threshold value,the missing value G3 is computed adaptively according to:

IF MAX(HCLAS, VCLAS) < THRESH R3 = (HPRED + VPRED)/2 ELSEIF VCLAS <HCLAS R3 = VPRED ELSE R3 = HPRED ENDThus, if both classifiers are smaller than the threshold value, anaverage of both predictor values is computed for R3. If not, then eitherHPRED or VPRED is used depending on which classifier HCLAS or VCLAS issmaller.

The missing blue values are interpolated in exactly the same way usingblue values in place of red. Once completed, the low-resolutionintermediate color image has been produced. From there, thelow-resolution color differences are computed as previously described.

The invention has been described in detail with particular reference tocertain preferred embodiments thereof, but it will be understood thatvariations and modifications are effected within the spirit and scope ofthe invention.

PARTS LIST 10 light from subject scene 11 imaging stage 12 lens 13neutral density filter 14 iris 16 brightness sensor 18 shutter 20 imagesensor 22 analog signal processor 24 analog to digital (A/D) converter26 timing generator 28 image sensor stage 30 digital signal processor(DSP) bus 32 digital signal processor (DSP) memory 36 digital signalprocessor (DSP) 38 processing stage 40 exposure controller 50 systemcontroller 52 system controller bus 54 program memory 56 system memory57 host interface 60 memory card interface 62 memory card socket 64memory card 68 user control and status interface 70 viewfinder display72 exposure display 74 user inputs 76 status display 80 video encoder 82display controller 88 image display 100 minimal repeating unit for Bayerpattern 102 repeating unit for Bayer pattern that is not minimal 110spectral transmission curve of infrared blocking filter 112 unfilteredspectral photoresponse curve of sensor 114 red photoresponse curve ofsensor 116 green photoresponse curve of sensor 118 blue photoresponsecurve of sensor 120 first green cell 122 red cell 124 blue cell 126second green cell 202 low-resolution partial color block 204high-resolution panchromatic block 206 low-resolution panchromatic block208 low-resolution color differences block 210 high-resolution colordifferences block 212 high-resolution final image block 220 first greencell 222 green pixels in first green cell 224 red cell 226 blue cell 228second green cell 230 upper pixel values for interpolating missing greenvalue 232 left pixel values for interpolating missing green value 234pixel with missing green value 236 right pixel values for interpolatingmissing green value 238 lower pixel values for interpolating missinggreen value 240 left pixel values for interpolating missing red value242 pixel with missing red value 244 right pixel values forinterpolating missing red value 250 upper pixel values for interpolatingmissing red value 252 pixel with missing red value 254 lower pixelvalues for interpolating missing red value 260 upper pixel values forinterpolating missing red value 262 left pixel values for interpolatingmissing red value 264 pixel with missing red value 266 right pixelvalues for interpolating missing red value 268 lower pixel values forinterpolating missing red value

1. A method for forming a digital color image comprising: capturing animage using a solid state image sensor having a pixel array thatincludes both panchromatic pixels and color pixels, wherein the colorpixels correspond to at least two color photoresponses and wherein thepixel array is made up of a plurality of tiled minimal repeating units,each minimal repeating unit including a plurality of cells, each cellincluding at least two panchromatic pixels and at least two pixelshaving the same color photoresponse; grouping signals from all of thecolor pixels in each cell into one or more group signals to increasecolor photosensitivity, wherein all pixels whose signals are groupedinto a group signal have the same color photoresponse; combining thegroup signals to form a low-resolution digital color image having afirst resolution based on the resolution of the cells; using secondsignals from the individual panchromatic pixels to provide ahigh-resolution digital panchromatic image having a second resolutionbased on the resolution of the pixels in the image sensor; and combiningthe high-resolution digital panchromatic image and the low-resolutiondigital color image to provide a high-resolution digital color image. 2.The method of claim 1, wherein each minimal repeating unit includes atleast twelve pixels.
 3. The method of claim 2, wherein the cells arenon-overlapping cells.
 4. A method for processing a color image capturedby an image sensor to permit the effective reproduction of a color imagecaptured under low lighting conditions, comprising: (a) using a solidstate image sensor to capture an image, the image sensor having a pixelarray including both panchromatic pixels and color pixels, wherein thecolor pixels correspond to at least two color photoresponses, andwherein the pixel array is made up of a plurality of tiled minimalrepeating units, each minimal repeating unit including a plurality ofcells, each cell including at least two panchromatic pixels and at leasttwo pixels having the same color photoresponse; (b) grouping signalsfrom all of the color pixels within each cell into one or more groupsignals to increase color photosensitivity, wherein all pixels whosesignals are grouped into a group signal have the same colorphotoresponse; (c) combining the group signals to form a low-resolutiondigital color image having a first resolution based on the resolution ofthe cells; (d) using second signals from the individual panchromaticpixels to provide a high-resolution digital panchromatic image having asecond resolution based on the resolution of the pixels in the imagesensor; and (e) combining the low-resolution digital color image and thehigh-resolution digital panchromatic image to produce a high-resolutiondigital color image.
 5. The method of claim 4, wherein element (b)includes summing the first signals from the plurality of pixels havingthe same color photoresponse in each group.
 6. The method of claim 4,wherein element (e) includes: summing pixel values from thehigh-resolution panchromatic image to produce a low-resolutionpanchromatic image having the first resolution; using the low-resolutionpanchromatic image and the low-resolution color image to produce a colordifference image; and using the color difference image and thehigh-resolution panchromatic image to produce a high-resolution colorimage.
 7. The method of claim 5 wherein element (e) includes: summingpixel values from the high-resolution panchromatic image to produce alow-resolution panchromatic image having the first resolution; using thelow-resolution panchromatic image and the low-resolution partial colorimage to produce a color difference image; and using the colordifference image and the high-resolution panchromatic image to produce ahigh-resolution color image.